1. Field of the Invention
The present invention relates to a focal point detection apparatus to be used in a single lens reflex camera or the like.
2. Related Background Art
As a focal point detection apparatus provided with a plurality of focal point detection areas, there is generally known an apparatus of a phase shift detection system as shown in FIGS. 11 and 12. FIG. 11 is a perspective view for showing a schematic configuration of a conventional focal point detection apparatus, and FIG. 12 is a view for schematically shown optical paths of the focal point detection apparatus 49 shown in FIG. 11, seen along the axis y in FIG. 11. Light flux from an object comes into the vicinity of a view field mask 12 through an unrepresented photographing lens to form an image there. Lens units 43b, 43c of a condenser lens 43 are arranged such that the optical axes of the respective lens units are eccentric with respect to the view field apertures 12Ya, 12Yb, and the optical axes L4a, L4b of the lens units 43b, 43c are parallel with the optical axis L of the entire optical system of the photographing lens. Out of light fluxes from the object field, a light flux passing the view field aperture 12X which takes a rectangular form elongated along the axis y is divided through the lens unit 43a, like chief rays r4a, r4b shown in FIG. 12, and through stop apertures 44a, 44b of an aperture mask 44 disposed at a position substantially conjugate with an exit pupil of the photographing lens and re-imaging lens units 45a, 45b of a re-imaging optical system 45, thereby forming images respectively on light receiving element arrays 46a, 46b of a CCD chip 46.
It is possible to detect a focal point control condition of the photographing lens by photo-electrically converting a pair of object images formed on the light receiving element arrays 46a, 46b. Specifically, when the focal point control condition of the photographing lens is in a so-called in-focus state in which a clear image is formed on a plane equivalent to a film, the paired object images on the light receiving element arrays 46a, 46b are imaged at positions with a predetermined distance between a front focus state and a rear focus state. Also, when the focal point control condition of the photographing lens is in the so-called front focus state in which the image is clearly formed in front of the plane equivalent to a film, the paired object images on the light receiving element arrays 46a, 46b are formed at positions having a first distance therebetween which is shorter than the predetermined distance. On the other hand, when the focal point control condition of the photographing lens is in the so-called rear focus state in which the clear image is formed in the rear of the plane equivalent to a film, the paired object images on the light receiving element arrays 46a, 46b are formed at positions having a second distance which is longer than the predetermined distance. Accordingly, these paired object images are photo-electrically converted into electric signals by the light receiving element arrays 46a, 46b, and a correlative arithmetic operation or the like is performed based on these electric signals, like in a shift amount detection apparatus disclosed in Japanese Patent application Laid-Open No. 60-37513, to obtain an amount of relative positional shift between the paired object images, whereby the focal point control condition of the photographing lens in a focal point detection area corresponding to the view field aperture 12X is detected.
Also, out of light fluxes from the object field, a light flux passing the view field aperture 12Ya which takes a rectangular form elongated along the axis y of the view field mask 12 is divided through the lens unit 43b, like chief rays r4c, r4d shown in FIG. 12, and through stop apertures 44c, 44d of the aperture mask 44 and re-imaging lens units 45c, 45d of the re-imaging optical system 45, thereby forming images respectively on light receiving element arrays 46c, 46d of the CCD chip 46. In the same manner, a light flux passing the view field aperture 12Yb is divided through the lens unit 43c, like chief rays r4e, r4f shown in FIG. 12, and through stop apertures 44e, 44f of the aperture mask 44 and re-imaging lens units 45e, 45f of the re-imaging optical system 45, thereby forming images respectively on light receiving element arrays 46e, 46f of the CCD chip 46. Then, the focal point conditions of the photographing lens are detected in the focal point detection areas out of the optical axis corresponding to the view field aperture 12Ya by use of the paired object images on the light receiving element arrays 46c, 46d and corresponding to the view field aperture 12Yb by use of the paired object images on the light receiving element arrays 46e, 46f.
In this manner, it is rendered possible to detect the focal point in a plurality of focal point detection areas.
Generally, for the focal point detection in a plurality of focal point detection areas, on a light receiving element array consisting of a CCD chip corresponding to a certain focal point detection area, a light (a so-called stray light) from another focal point detection area is incident, to give a great influence on the accuracy in the focal point detection. For this reason, the art of shielding this stray light by use of a light shielding member is disclosed in the Japanese Patent Application Laid-Open No. 63-289513.
FIG. 13 is a view for schematically showing the optical paths of the focal point detection apparatus 49 shown in FIG. 11 seen along the axis y in this drawing, plus a light shielding member. Note that the identical members to those in FIGS. 11 and 12 are given the same numerals and description thereof will be omitted. A light flux R41 passing through the view field aperture 12X is a focal point detection light flux for entering the light receiving element array 46a through the stop aperture 44a, while a light flux R42 passing through the view field aperture 12Yb is a focal point detection light flux for entering the light receiving element arrays 46e, 46f through the stop apertures 44e, 44f. In this case, a light flux R43 which passes through the stop aperture 44a also transmits through the view field aperture 12Yb. This light flux R43, however, enters the light receiving element array 46a which is an undesired light receiving element array. In other words, the light flux R43 becomes a stray light. Therefore, in order to prevent such stray light, a light shielding member m41 is provided. However, out of the light fluxes transmitting the field view aperture 12Yb, a stray light such as the light flux R44 which passes through the stop aperture 44b can not be completely shielded by the light shielding member m41. Also, the light shielding member m41 is formed to have holes at positions for passing the focal point detection light fluxes, like the view field mask 12, so that, in order to completely shield the light flux R44, a light shielding member having a more complicated form is required.
In addition, when a plurality of focal point detection areas are provided in a photographing frame, a pair of stop apertures and a pair of re-imaging lens units for forming images of a pair of objects on light receiving element arrays are required for each of the focal point detection areas. Then, there is disclosed in the Japanese Patent No. 2663657 a focal point detection apparatus having a focal point detection area at the center of the photographing frame, an aperture mask for using in common stop apertures for focal point detection areas on the both sides of the above-mentioned focal point detection area around the photographing frame, and a re-imaging optical system for using re-imaging lens units in common. FIG. 15 is a perspective view for showing a schematic configuration of the focal point detection apparatus disclosed in the Japanese Patent No. 2663657. In this focal point detection apparatus 39, a light flux which passes through a view field aperture 32Xa corresponding to a focal point detection area at the center of a photographing frame forms an image on a CCD chip 36 through stop apertures 34c, 34d of an aperture stop 34 and a re-imaging optical system 35. In the same manner, a light flux which passes through a view field aperture 32Xb corresponding to the focal point detection area at the center of the photographing frame forms an image on the CCD chip 36 through stop apertures 34a, 34b of the aperture mask 34 and the re-imaging optical system 35. A light flux which passes through a view field aperture 32Ya corresponding to a focal point detection area on the periphery of the photographing frame forms an image on the CCD chip 36 through the stop apertures 34a, 34b of the aperture mask 34 and the re-imaging optical system 35. In the same manner, a light flux which passes through a view field aperture 32Yb corresponding to a focal point detection area on the periphery of the photographing frame forms an image on the CCD chip 36 through the stop apertures 34a, 34b of the aperture mask 34 and the re-imaging optical system 35. That is, the respective light fluxes passing through the view field apertures 32Xb, 32Ya, 32Yb use the stop apertures 34a, 34b of the aperture mask 34 in common.
In general, when a distance between a pair of stop apertures of an aperture mask for one focal point detection area is widened, the accuracy in the focal point detection is enhanced. However, a focal point detection area positioned farther from the center of the photographing frame has a larger vignetting phenomenon of the photographing lens. This vignetting phenomenon is described below with reference to FIGS. 14A and 14B. FIG. 14A is a view of a photographing lens when the optical axis of the photographing lens and the visual line of the observer coincide with each other, seen from the exit side of light flux. In this case, the exit pupil P looks like a circle. FIG. 14B is a view of the photographing lens when the observer takes a look in a slanting direction with respect to the optical axis of the photographing lens, seen from the exit side of the light flux. In this case, the exit pupil P' is formed to be a partly cut-away circle, which is called vignetting. When this vignetting becomes further larger, an area Q for indicating a pair of stop apertures is vignetted. That is, when a focal point detection area is located farther from the center of the photographing frame, the focal point detection is affected more easily by the vignetting caused by the light flux from the object field, unless the distance m3 between the paired stop apertures is reduced, which may impede the focal point detection.
For this reason, a focal point control condition detection apparatus is disclosed in the Japanese Patent Application Laid-Open No.1-288810, in which a focal point detection area located farther from the center of the photographing frame has a smaller stop aperture of the aperture mask, or a pair of stop apertures with a narrower gap therebetween. That is, such focal point control condition detection apparatus has a higher accuracy in the focal position detection in the vicinity of the center of the photographing frame which is used most frequently, and, at the same time, can perform focal point detection at a plurality of positions in the photographing frame. However, in the focal point control condition detection apparatus disclosed in the Japanese Patent Application Laid-Open No. 1-288810, a pair of stop apertures and a re-imaging lens unit are required for each of the plurality of focal point detection areas, which results in an increased size of the focal point detection apparatus.
In the focal point detection apparatus disclosed in the Japanese Patent No. 2663657, since the light fluxes passing through the view field apertures 32Xb, 32Ya and 32Yb corresponding to the focal point detection areas located at the center of the photographing frame and on the periphery thereof use the stop apertures 34a, 34b in common, as shown in FIG. 15, increase in size of the focal point detection apparatus can be avoided. However, in the focal point detection area at the center of the photographing frame, though there is a room for enhancing the accuracy in focal point detection by expanding the distance between the corresponding paired stop apertures 34a, 34b, since the stop apertures 34a, 34b are used in common with the focal point detection areas on the periphery of the photographing frame, the distance between the paired stop apertures is determined taking into consideration that there is no influence of the vignetting of the photographing lens in the focal point detection areas on the periphery of the photographing frame. That is, the distance between the paired stop apertures in the focal point detection area at the center of photographing frame is the same as that distance of a paired stop apertures in a focal point detection area on the periphery of the photographing frame. Accordingly, there arises a problem that the proper accuracy in the focal point detection can not be fully displayed.
Further, in the Japanese Patent Application Laid-Open No. 2-50115, there is disclosed a focal point detection apparatus in which focal point detection areas are arranged on the diagonal lines of the photographing frame.
As described above, if a plurality of focal point detection areas are provided in a photographing frame, a pair of stop apertures and a pair of re-imaging lens units for forming a pair of object images on light receiving element arrays are required for each of the focal point detection areas. Also, in a focal point detection area by a view field aperture having a rectangular form elongated along the axis y, the focal point detection can not be performed for an object with a brightness fluctuation along the axis x.
In the Japanese Patent Application Laid-Open No. 2-50115, focal point detection areas are disposed also on the diagonal lines of the photographing frame for allowing the focal point detection at a larger number of positions by increasing the number of the focal point detection areas. In this arrangement, however, a pair of stop apertures and a pair of re-imaging lens units are required for each of the additional focal point detection areas. Also, in a focal point detection area corresponding to a cross-shaped view field aperture, two pairs of aperture masks and two pairs of re-imaging lens units are required, which results in the increased size of the focal point detection apparatus and a more complicated structure of the focal point detection optical system itself.